How to define and identify lessons learned?

Dear EvalForward colleagues, As M&E professionals, we are often requested to identify and consolidate “lessons learned”. From my experience, there is not a shared understanding/ definition of what a lesson learned is. Often I come across lessons that sound like findings, conclusions, or sometimes even like recommendations and therefore belong somewhere else. I would be interested to learn from this community: which is the definition of lesson learned that you adopt or one you find useful?

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How to define and identify lessons learned?